Gas spectroscopy measures the absorption of light by a gas sample. The absorption of a given wavelength of light can be measured to determine the concentration of a gas of interest in the sample.
In tunable diode laser absorption spectroscopy (TDLAS) a sample of the gas of interest in a reference cell as a wavelength reference is used to keep the laser line-locked to the linecenter of the absorption feature of interest. In frequency modulation spectroscopy (FMS) the laser is modulated across the absorption feature and the resulting signal is expanded in a Fourier Series. The coefficients of the expansion are denoted harmonics. The even harmonics each exhibit a maximum and the odd harmonics each exhibit a zero-crossing at the linecenter. Line-locking the laser to the feature of interest is accomplished by monitoring the third harmonic of the gas in the reference cell.
The sensitivity of TDLAS systems is often limited by interference patterns attributable to the optics of the system and by fluctuations in laser intensity, and more importantly by fluctuations in background levels of the second harmonic signal. Various prior art methods of eliminating interference patterns include mechanical approaches, specialized modulation waveforms, specialized modulation frequencies and multiple modulation frequencies. Fluctuations in second harmonic signals have been compensated by splitting the laser beam before the sampling cavity, and projecting one portion of the laser beam through the sampling cavity to a first detector and a second portion of the laser beam directly to a second detector. The second harmonic signal from each of these detector is then nomalized by dividing by the corresponding DC levels. In this manner, both fluctuations in laser intensity and second harmonic signal may be removed from the measurement. The paths of the first and second portions of the laser beams in such systems is typically significantly different.
In the past, TDLAS systems have typically extracted a sample for measurement into a remote measuring device. This approach requires pumps, filters and heated supply lines, adding complexity to and decreasing reliability of such a system. The accuracy of such remote measurement systems may be limited by absorption, desorption, precipitation or chemical reaction of the gas of interest in the delivery system.
An in situ measurement apparatus can be used to avoid the problems of extracting a sample and to measure the gas in an unperturbed environment. In an in situ measurement apparatus, the sampling cavity is mounted in the flow of gases in a stack or duct. Gas diffuses into the sampling cavity through filters that prevent particulates from entering the cavity. Particulates tend to settle on the reflective surfaces in the cavity and degrade the signal or damage the optical surfaces. In the past, ceramic filters were used; however, the porosity of ceramic filters is not easily controlled and the ceramic filters were difficult to integrate into the metal structure of the sampling cavity.
Previous in situ systems did not use TDLAS. These systems typically used a single reflector in the sampling cavity which limited the path length of the light beam in the sampling cavity and the accuracy of the apparatus.